1. Field of the Invention
The present invention relates to a gear structured to reduce backlash.
2. Description of the Related Art
Compared to a metal gear, a plastic gear has operational advantages such as being light in weight, being self-lubricating, thereby allowing use without a lubricant, and having low noise. At the same time, the plastic gear has manufacturing advantages such as allowing high productivity and allowing mass-production, thereby allowing a reduction in manufacturing costs. In recent years, the plastic gear having such advantages has been widely used in, for example, gear trains in ink-jet printers, power transmission sections of automobiles, and gear trains in precision machinery. The range of use of the plastic gear is expanding.
Even in the plastic gear having low noise, as described above, teeth-knocking noises (undesirable noises) frequently occur depending on usage methods, causing undesirable noises. For example, when the plastic gear is used in an intermittent rotation transmission mechanism, two gears that are intermeshed transmit power while frequently repeating rotation-start and rotation-stop. Therefore, at rotation-start, a tooth of a gear on a driving side immediately after the rotation-start collides with a tooth of a gear on a driven side that is not moving. As a result, the teeth-knocking noise occurs. At the same time, at rotation-stop, the tooth of the gear on the driven side collides with the tooth of the gear on the driving side that has stopped before the gear on the driven side, as a result of inertia. As a result, the teeth-knocking noise occurs. In the intermittent rotation transmission mechanism, rotation-start and rotation-stop are frequently repeated. Therefore, the teeth-knocking noises frequently occur and become undesirable noise.
To prevent such teeth-knocking noises during power transmission, innovative plastic gears have been proposed.
For example, a plastic gear disclosed in Patent Literature 1 includes an almost triangular cavity section on both end faces of a tooth in a tooth-width direction. The cavity section reduces the thickness of the tooth. An angular thin-walled section is formed surrounding the cavity section. The gear is structured so that, as a result of differential shrinkage after injection-molding, the tooth thickness of the thin-walled section is thicker than the tooth thickness of an area without a thin-walled section. In other words, the thin-walled section is formed so as to project from a tooth flank in the area without the thin-walled section. In the gear, the thin-walled section is flexibly deformed during an initial stage of meshing with a partner gear and absorbs impact. Therefore, the gear can prevent the teeth-knocking noises from occurring.
Patent Literature 1: Japanese Patent Laid-open Publication No. 2003-90412